High impact polystyrene polyblends (HIPS) comprising polystyrene having a rubber phase dispersed therein, as crosslinked rubber particles, are known. Historically, mechanical blends were prepared by melt blending polystyrene with raw rubber which was incompatible and dispersed as crosslinked rubber particles to reinforce and toughen the polymeric polyblend. More recently, HIPS polyblends have been prepared by mass polymerizing solutions of diene rubber dissolved in styrene monomer in batch reactors wherein the rubber molecules were grafted with styrene monomer forming polystyrene polymer grafts on the rubber along with polystyrene polymer in situ in the monomer. As the polystyrene-monomer phase increases during polymerization the grafted rubber phase inverts readily as rubber particles comprising grafted rubber and occluded polystyrene contained therein with said particles crosslinked to maintain the rubber particles as discrete particles dispersed in the polystyrene which forms a matrix phase of the HIPS polyblend.
U.S. Pat. No. 3,488,743 teaches HIPS polyblends prepared with polybutadiene rubbers having a range of molecular weights and prefers a range of from about 30,000 to 110,000 when polymerized in solution with polymonovinylidene polymers being present during polymerization, providing a balance between impact strength, toughness and gloss. Commercial rubbers are taught to range from about 120,000 to 250,000 with impact strength being maximized at about 150,000 when polymerized in solutions with the monomers alone. U.S. Pat. No. 3,311,675 teaches methods of preparing HIPS polyblends using diene rubbers and indicates that the diene rubbers usually have a molecular weight of 15,000 and higher with no definition of a preferred molecular weight range to maximize toughness.
Engineering uses of HIPS polyblends require improved toughness where load bearing properties are needed in automotive and appliance applications. It has been discovered that polybutadiene rubbers having a higher molecular weight and broader molecular weight distribution than the commercial rubber used by the prior art, provide such rubber reinforced polyblends with greater toughness.
An objective of the present invention is to provide HIPS polyblend compositions with greater toughness.
Another objective is to provide HIPS polyblends toughened with particular polybutadiene rubbers having higher molecular weights and broader molecular weight distributions.